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66 DESIGN007 MAGAZINE I FEBRUARY 2024 Article by Chris Keirstead PFC FLEXIBLE CIRCUITS An important but sometimes overlooked aspect of flex and rigid-flex fabrication and assembly is the flex circuit tail, which is attached to a rigid PCB with pressure-sensitive conduc- tive adhesives. is sub-assembly is becoming very common. We oen see this applied to glass displays and microelectronic applications. But this method of attachment without con- nectors is not as straightforward as one may think. e wide varieties of circuitry to be bonded create many attachment challenges. e best track configuration is not always pos- sible, so one must be familiar with the optimum layouts as well as what is not recommended when considering these trade-offs. Less than optimum trace and pad layouts result in the need for more customization and smaller pro- cessing window variations for the bonding process. Here are some common configurations that flex assemblers have developed, and the pre- ferred approach for the method of bonding. Interposer Effect and Co-planarity Adjustment Bonding imbalance results from either bad co-planarity between the bonder head and stage or a thickness deviation of the flex and PCB materials. e interposer will absorb these discrepancies to some extent and help bring the